662	RENAISSANCES
when this fatal flaw in his efficiency was discovered and exploited by
the Cossacks;1 and the rare amphibious prowess of 'Water Sakas', who
turned to account on the Indus a fluvial waterman's skill that they
had acquired on the Oxus,3 was manifestly one of those exceptions that
prove a rule. If this reading of the historical evidence is correct, it
requires us to discard, as a misleading illusion, the visual impression
produced by a synoptic view of the physiographical maps of the Han
Empire and the Roman Empire when we place these two pictures side
by side. If, in ignorance of the historical facts, we were to ask ourselves
which of these two strikingly diverse physiographical structures might
be expected to offer the greater facilities to mounted invaders, the
obvious a priori answer would be that a continental empire built round
the basins of two rivers with a common watershed would be as easy
for an invading cavalry to overrun as it would be difficult for these
horsemen to conquer the transmarine provinces of a maritime empire
built round the basins of two interconnecting bays of one continuous
inland sea. Yet this a priori answer is given the lie by the historical
fact that the Alano-Vandals brilliantly succeeded in conquering a trans-
marine North-West Africa in a.d. 429-39, whereas the Hunno-Tibetans
were ignominiously repulsed in a.d. 383 on the banks of the River Huai.
The outcome of the contest between an Alano-Vandal attack and
a Roman defence in North-West Africa in the fourth decade of the
fifth century of the Christian Era was decided, as we have already
observed, by the cumulative effect of one feature of the Roman Empire's
physiographical structure and one politico-military legacy of an ante-
cedent chapter of Roman imperial history. The Alano-Vandals were
sped on their way from Central Europe to North-West Africa by the
conductivity of the invaded oecumenical empire's central sea; but their
subsequent conquest of Rome's North-West African dominions would
certainly not have been completed so quickly, and might perhaps never
have been completed at all, if, in the course of the 575 years that had
elapsed, by the date of the Alano-Vandal landing in Africa in A.D. 429,
since Rome's first annexation of territory in Africa in 146 b.c., the
Romans had emulated the thoroughness of the Prior Han empire-
builders' work in Southern China by effectively subjugating all the
North-West African highlands up to the natural frontiers afforded by
this virtual island's Saharan desert coasts. The conjuncture that ensured
the rapid and complete extinction of Roman rule in North-West Africa
after the advent of the Alano-Vandals in a.d. 429 was the survival of
a still unsubjugated local barbarian enemy at the gates, ready to join
hands with the exotic Eurasian barbarian new arrivals. The alliance
between the interloping horsemen and the indigenous highlanders was
as decisive as it was inevitable.
Thus Rome failed to furnish a senile Hellenic World with a defensible
fortress in a North-West Africa which, in the physiographical structure
of the Roman Empire, was the morphological counterpart of the New
South in the structure of the Han Empire. On the other hand, Rome
did succeed in creating an imperfect, yet locally effective, functional
i See II. ii. 154-7 and V. v. 283 and 313-15.	2 See V. v. 603.